A digital cellular communication system uses an RF transmitter to modulate a radio carrier with the baseband coded data for transmission. The baseband coded data is a digitally modulated waveform. Through an I/Q modulator, the baseband data modulates a radio frequency (RF) carrier as set by a local oscillator. The transmitter also has a bandpass filter, such as an SAW or BAW device, to filter out spurious frequencies to reduce out-of-band noise, and a driver amplifier to strengthen the RF modulated signal. Finally, a power amplifier is used to convey the modulated signal to the antenna for transmission.
The efficiency of the power amplifier can be improved by modifying the power amplifier supply voltage in accordance with the envelope of the modulated RF signal. This technique is known as envelope tracking, and the power amplifier is also referred to as a dynamic supply voltage amplifier. Modification of the supply voltage can be carried out with an analog control or a digital control. As disclosed in Ranjan et al. (“Microwave Power Amplifiers with Digitally-Controlled Power Supply Voltage for High Efficiency and High Linearity”, IEEE MTT-S International Microwave Symposium Digest, pp. 493-496, vol.2) and Hanington et al. (“High-Efficiency Power Amplifier Using Dynamic Power-Supply Voltage for CDMA Application”, IEEE Trans MTT, vol. 47, No. 8, pp.1471-1476, 1999), a diode is coupled to the driver amplifier to detect the envelope of the RF modulated signal. The detected envelope, which is an analog signal, is then used to vary the supply voltage of the power amplifier. While this prior art envelope tracking technique significantly improves the efficiency of the power amplifier in an RF transmitter, it does not deal with the rapid gain and phase changes when the supply voltage is very low. Rapid phase changes result in high phase distortion, for example. It is possible, however, to compensate the rapid gain and phase changes by pre-distorting the I and Q signals going to the I/Q modulator. Ranjan et al. uses a digital signal processor to generate a voltage control based on the I and Q values. While this approach somewhat improves the efficiency of the power amplifier, it relies on a complicated algorithm for pre-distorting the I and Q values. However, both the digital approach and the analog approach, as disclosed in Ranjan et al., fail to address the problems associated with the transmit slot ramp-up and ramp-down between slots in the slotted operation in a mobile radio design, such as that in the TDMA and CDMA systems.
Thus, it is advantageous and desirable to provide a method and device for further improving the envelope tracking technique.